Advanced Marine Electrics and Electronics Troubleshooting A Manual for Boatowners and Marine Technicians

at the farthest point from the source of the sig-
nal. The unit can only tell you where the
bouncing stops; it can’t determine if it’s an open
circuit, a short circuit, or a complete circuit.
Thus, you really need to know how a circuit is
laid out and what components are installed on
it to correctly interpret the TDR’s results.

Using the TDR to Check Wire Size

In addition to finding faults in a circuit, you can
use a TDR to determine if a circuit complies
with ABYC standards for wire size. Here are the
facts we know about the circuit example above:

testing continuity and tracing circuits 43

The 3% voltage drop table from ABYC Standard E-11. (Courtesy ABYC)

• The navigation light being tested in photo 4
  opposite is working.


• The 12 V circuit is wired with 14 AWG wire.


• The total length of the circuit from the
  panel to the light and back to the source is
  61.2 feet (30.6 feet 2).


• The lightbulb draws 2 amps.

Navigation lights are a critical circuit, and

the ABYC (and U.S. Coast Guard) permits a

maximum 3% voltage drop. We then refer to

the appropriate voltage drop table in ABYC

Standard E-11, reproduced below.